Electrochemical recovery of sulfuric acid

ABSTRACT

Electrochemical process for purification and concentration of used sulfuric acid containing hydrocarbon wherein water is added to bring the water content of the acid to between one and two times the stoichiometric amount of water in relation to the amount of carbon present, and electrochemically treating the diluted acid with a current density in the range of 200-2,500 amperes per square foot to cause oxidation of the carbon to carbon dioxide at the anode. Process is particularly applicable to the recovery of used sulfuric acid alkylation catalyst.

Unite States atent 72 inventor WilliamB.Mather,Jr. 2,793,182 5/1957 Visnapuu .L 204/130 HopewellJunction,N.Y. 2,744,861 5/1956 Thomas 204/130 211 AppLNo. 754,555 1,630,074 8/1924 Rogersetal. 204/136 [221 Filed 2 FOREIGN PATENTS [45] Patented Oct. 6,1 1 [73] Assigns Texmlnc 586,878 11/1959 Canada 204/130 New York, N.Y. Primary Examiner- Howard S. Williams ELECTROCHEMICAL RECOVERY OF SULFURIC Assistant Examiner-R. L. Andrews Attorneys-K. E. Kavanagh. Thomas H. Whaley and H. L.

Madinger ABSTRACT: Electrochemical process for purification and concentration of used sulfuric acid containing hydrocarbon wherein water is added to bring the water content of the acid to between one and two times the stoichiometric amount of water in relation to the amount of carbon present, and electrochemically treating the diluted acid with a current density in the range of ZOO-2,500 amperes per square foot to cause oxidation of the carbon to carbon dioxide at the anode. Process is particularly applicable to the recovery of used sulfuric acid alkylation catalyst.

ELECTROCHEMICAL RECOVERY OF SULFURIC ACID BACKGROUND OF THE INVENTION More particularly it relates to an electrochemical method for 1 the recovery of sulfuric acid.

2. Description of the Prior Art It is broadly old to recover or regenerate sulfuric acid by electrochemical means, but heretofore sufficiently good results have not been obtained by such means for them to be of commercial significance. In US Pats. Nos. 2,793,180 issued July 16, 1953 to Philip S. Fay and Naima Visnapuu, 2,783,181 issued July 16, 1953 to Naima Visnapuu and 2,793,182 issued July 16, 1953 to Naima Visnapuu a process is described which is mainly electrodialytic with the impurities being drawn into the cathode compartment by electrodialysis. An important feature of the feature of the apparatus for this process is the presence of a porous diaphragm which separates the cathode and anode compartments. In this prior art process the concentration of the acid in the anode compartment is increased while the concentration of the acid in the cathode compartment is decreased. Thus, two products are in reality formed in about equal amounts: one an acid of relatively low concentration and in which the impurities are concentrated and two an acid of relatively high concentration and containing a decreased amount of impurities. While the impurities tend to concentrate in the lower strength acid in the cathode compartment there is very little, if any, actual removal of impurities by this process. lmpure or used acids are used as they occur without any pretreatment, such for example as used alkylation acid, which is of primary interest. In general special precautions are required to prevent foaming and sludging of the acid. Used alkylation acid usually contains about 25 percent by weight of water and about the same quantity of hydrocarbon.

SUMMARY OF THE INVENTION In the process of my invention, acid such as used alkylation acid is adjusted in water content so that the amount of water present is about 3-6 times by weight of the amount of carbon present, or that amount of carbon which it is desired to remove. The carbon content of the organic matter in the acid is oxidized in the anode compartment to carbon dioxide and hydrogen is formed in the cathode compartment. Although the actual oxidizing agent probably is peroxydisulfuric acid the apparent effect is as though water is the oxidizing agent and water as well as hydrocarbon is removed.

By insuring that sufficient water is present, the desired oxidation reactions proceed as desired. The water also prevents the formation of free sulfur trioxide which ordinarily is undesirable. lFuming acid containing free sulfur trioxide can be produced if the reaction is continued after all of the water is consumed.

In my method a porous diaphragm is not required to separate the cathode and anode compartments as in the methods of the prior art. The water content of the acid is increased so that it is about l-2times the stoichiometric amount required to oxidize the carbon in the hydrocarbon material to carbon dioxide. The stoichiometric amount is 3 parts by weight of water to one part by weight of carbon. During the process both water and hydrocarbon are removed, consequently, the yield of a single recovered product is very high, and if desired the concentration of the recovered acid can reach 100 percent, or even fuming.

An advantage of my invention is that a single acid product in high yield and in high concentration can be obtained. If desired, essentially 100 percent acid can be produced.

Another advantage is that high current densities can be used. Another advantage is that foaming and sludging are not problems in my process.

Still another advantage is that a porous membrane is not required although one can be used.

BRIEF DESCRIPTION OF THE DRAWING Having set forth the general nature of the invention, it will be best understood from the following more detailed description and the accompanying drawing showing diagrammatically an electrolytic cell for carrying out the process of the invention. 0

DESCRIPTION OF A PREFERRED EMBODIMENT In the drawing, electrolysis cell 11 consists of an enclosed vessel containing a cathode compartment 12 in the form of a hollow cylinder which is open at the bottom permitting communication of the contents thereof with anode compartment 15 surrounding cathode compartment 12. A cathode element 13 which may be a carbon or graphite rod or other suitable anode material as will be discussed more fully hereinafter is axially located in cathode compartments 12 and a suitable anode 16 consisting of platinum foil or other suitable anode material, as will be discussed hereinafter, is located in anode compartment 15. Anode 16 and cathode 13 are connected by 19 and 20 respectively to DC power supply 21. Power supply 21 may consist of batteries but preferably is a regulated DC supply. Used or spent acid to be recovered is charged through line 10 into cathode compartment 12. Llne 14 is provided to release hydrogen gas formed in cathode compartment 13 and line 18 is provided to release carbon dioxide gas formed in anode compartment 15. Stirrer 17 is provided for agitation and mixing of the contents of cell 11. Stirrer 17 is attached to shaft 24 which passes through bushing 25 on the bottom of cell 11 and is connected to drive motor 26. Line 23 is provided for withdrawing recovered acid from the cell. The top of cell 1 l is sealed with closure 22.

The electrolysis cell may be operated with continuous addition of charge acid and continuous withdrawal of products. In a typical operation, used sulfuric acid alkylation catalyst containing 4.3 percent ofcarbon, 5.3 percent water and 89.5 percent sulfuric acid by weight is first diluted with l 1.9 percent by weight of water. The resulting total water content of the acid is equal to l'times the stoichiometric amount required to react with the carbon in the hydrocarbon contaminants. The diluted used alkylation acid is then charged through line 10 near the top but below the liquid level of the acid in cathode compartment 12. Electric current from DC power supply 21 is passed through the cell by means of anode 16 and cathode 13 to effect electrolysis of the acid. Hydrogen evolved as a result of electrolysis is withdrawn through line 14. Acid in cathode compartment 12 passes down through the compartment and out the bottom at a flow rate such that molecular hydrogen is not carried into the anode compartment 15. In the electrolysis reaction, peroxydisulfuric acid is formed at platinum foil anode 16 as a result of oxidation of sulfuric acid at the anode. Acid from cathode compartment 12 entering anode compartment 15 comes into contact with the peroxydisulfuric acid formed at anode 16 and the hydrocarbon content is oxidized. Mixing of the contents of the cell is provided by stirrer 17. 0xidation of the hydrocarbon content of the acid results in the formation of carbon dioxide which is evolved as a gas from anode compartment 15 through line 18. Recovered acid sub stantially free of carbon and hydrocarbon is passed from anode compartment 15 through line 23.

REACTIONS OCCURRING IN THE PROCESS The reactions occurring in the electrolytic cell are complex and are not completely understood. However, the following are believed to be reasonable and consistent with the results obtained and are presented for purposes of clarification only to assist in a better understanding of the invention without imposing any limitations or restrictions thereon.

In the anode compartment of the electrolysis cell sulfuric acid is believed to be oxidized to peroxydisulfuric acid as follows:

The peroxydisulfuric acid then oxidizes the hydrocarbon component of the used acid to carbon dioxide with regeneration of sulfuric acid, as follows:

The amount of electrical current required for the complete removal of carbon by the above equation, which assumes 100 percent efficiency, is 5.5 Faradays per gram atom of carbon. A Faraday is equal to 96,500 ampere seconds, or Coulombs, or 26.8 ampere hours. Thus, 1 gram atom (12 grams) of carbon requires 5.5 Faradays or l47.4 ampere hours for complete oxidation. Since less than l percent conversions efficiency is usually obtained in practice, for example 50 to 95 percent, the amount of current required is usually in the range of about 155 to 295 ampere hours, or 5.7 to II Faradays per gram atom of carbon. Current efficiency is the ratio as a percentage figure of the theoretical power requirement to the actual power consumed for the amount of carbon consumed.

The overall result of the various reactions involved in the electrolytic oxidation of hydrocarbon by water may be represented by equation:

CH2 ZHgO C0: 3H2

at at anode cathode Since hydrogen in the water and hydrogen in the hydrocarbon is evolved or removed from the acid as molecular hydrogen, it is desirable to insure that only sufficient water is present to ox idize the carbon in accordance with the reaction:

From this equation it can be calculated that each part of carbon requires 3 parts by weight of water for complete oxidation. In practice, only about 65 percent current efiiciency is generally obtained and in general this less than 100 percent current efficiency is thought to be a result of side reactions. One major possible side reaction is the oxidation of water rather than carbon by the peroxydisulfuric acid, as follows:

Thus, unless it is desired to make fuming sulfuric acid, more water than necessary should not be added as it will result in an increase in power consumption.

VARIABLES 1. Water content ofthe acid As indicated above, not only must enough water be present in the acid to react with the carbon, but there also should be enough water present to react with the peroxydisulfuric acid in the above side reaction. I have found that the amount of water present in the acid on a weight basis should be about 1 to 2 times the stoichiometric amount necessary to oxidize all of the carbon to carbon dioxide. If for any reason it is not desired to remove all of the carbon from the acid, then a lesser amount of water may be present. As a matter of fact, I have found that the final 0.1 to 0.2 of the carbon is the most difficult to remove in that it requires a longer electrolysis time and higher current. In general, the amount of current or power required is directly proportional to the quantity of carbon removed.

Although for substantially complete removal of carbon which is present as hydrocarbon, and to produce an acid of about 98.5-99.5 percent by weight H 80 I have found it advantageous to use about I to 2 times the stoichiometric amount of water required to oxidize the carbon to CO l prefer to operate in a range of about 1.0 to 1.8 times the stoichiometric amount. Expressed in moles, two moles of water are required for each mole of carbon, and I prefer a range of 2.0 to 3.6 moles of water for each mole of carbon. Expressed on a weight basis, three parts by weight of water are required for each part of carbon, and I prefer a range of about 3.0 to 5.4 parts by weight of water for each part of carbon.

Thus, an acid containing 3 percent by weight of carbon requires 9.0 to 16.2 percent by weight of water. If too much water is added or is present the acid will be weaker than desired when substantially all of the carbon is removed. If the electrochemical reaction is continued to remove the excess water, additional power is required. This of course adds to the operating cost. On the other hand, if too little water is added, free sulfur trioxide is formed, and ultimately produces sulfuric acid containing free sulfur trioxide or fuming acid, and this can occur before all of the carbon is removed.

2. Source of Acid Acid from any source containing hydrocarbon diluent can be recovered or purified by the process of my invention. However, since the amount of power consumed in proportional to the amount of carbon removed, as a practical matter my process is best suited for acid which is relatively low in organic matter and water. An ideal acid and one of great commercial interest is spent or used sulfuric acid catalyst from the alkylation of isobutane with olefins. Used alkylation acid usually contains about percent sulfuric acid and about 3-5 percent each of water and hydrocarbon. As indicated above, water must be added to such an acid for effective carbon removal.

Some used acids may contain considerably more water than is required for oxidation of the carbon during the recovery process. Such an acid might be used acid from the manufacture of isopropyl alcohol which acid ordinarily contains as much as about 4050 percent water and only about i-2 percent carbon by weight. When an acid is encountered which has an excess of water as relates to the amount of carbon present it is advantageous to remove the excess water by simple distillation prior to using my electrochemical process for removal of the carbon.

3. Mixing Oxidation of the carbon takes place throughout the anode section of the apparatusv Therefore, it is desirable that Sufficient mixing be employed to provide a fairly uniform mixture. On the other hand, the mixing should be so designed and conducted that hydrogen from the cathode compartment cannot escape into the anode compartment.

4. Power Requirements and Electrodes The electrodes in the electrolysis cell should be reasonably resistant to the action of sulfuric acid and to oxidation under the conditions of operation and should not be appreciably soluble in the acid system. The anode must be electrically conductive under anodic conditions and similarly the cathode must be electrically conductive under cathodic conditions. In general, anode materials tend to dissolve to some extent in the acid, some more than others. However, the rate of solution for platinum iridium, lead and lead dioxide is quite slow and these materials are considered to be quite satisfactorily as anode materials. Lead, graphite. carbon and tantalum are satisfactory materials for use as cathodes, with graphite being preferred. Nickel and platinum are not satisfactory as cathodes, as they reduce sulfates to free sulfur. Mercury also is not a satisfactory cathode as it reduces sulfate to sulfide.

In the operation of the electrolysis cell, the electrodes, and the anode in particular, tend to become smaller in size with use due to gradual solution of the electrode material in the acid. In addition, the power consumption increases as the current density (electrolysis current divided by anode surface area) increases. Furthermore, the voltage drop between electrodes increases as the surface area of the electrodes decreases and therefore the applied voltage required to maintain a given current density also increases with operating time. In general, the applied voltage will be within the range of from about 2 volts to as high as about 25 volts.

When operating with a low-cost electrode such as lead it generally is preferably to employ a relatively low current density of the order of from about 20 to 2,500 amperes per square foot, with current densities in the range of from 200 to 500 amperes per square foot being preferred. when using a highcost electrode such as platinum, a high current density is preferred with current densities ranging from 500 to 2,500 amperes per square foot or higher being satisfactory.

5. Temperature The current efficiency, or efficiency of the reaction, increases with temperature, apparently due to the greater oxidizing power of peroxydisulfuric acid at elevated temperaover about 0.5 percent carbon and 1.0 percent water. Such an acid is in the range of about 98.5-99.5 percent concentration and has quite low carbon and water contents.

The results of the examples demonstrate the flexibility of ture. For example, experiments carried out in the 30 to 70 F. 5 my process. Within reason, an acid of almost any water conrange resulted in a current efficiency of approximately 60 pertent and carbon content can be produced by my process, for cent while experiments carried out in the 90-160 F. range example, water contents in the range of to 12 percent and had a current efficiency ofapproximately 75 percent. A range f contents the range of 0 P But as of 30 to 175 F. has been found to be satisfactory, and I prefer dlcated Y PmCeFS of most mtfz'rest l recovermg to Operate at a temperature of about In some cases used alkylation acid to give a product quite low in water and however, for practical purposes it may be advantageous to comemsoperate at about the same temperature as the temperature of obvlously) many modlficauons and vanamlns of the f the acid as received at the recovery unit. 1n the case of used as herelflhefm Set forth may be made depamng acid from an alkylation process this would be about 3550 F. r m t splm and SCPPe thereof therefor only such if no heating or cooling of the acid occurred en route to the hmltanons,should be Imposed as are md'cated m the recovery unit. 'i claims lclaim: 1. An electrolysis process for the reduction of hydrocarbon EXAMPLES content of used sulfuric acid alkylation catalyst containing 0 hydrocarbon contaminants in an electrolysis cell having a In table 1 data are presented for four electrochemical acid Cathode Compartment containing a cathode element and hay. remvery examples carried out in accordance with the ing an open lower portion in direct communication with an teimhhlgs of invention in apparatus of the yp Show" in anode compartment containing an anode element, said the drawing and described in detail hereinbefore. In these excathode compartment b i Substantially isolated f Said amples the ratio of the quantity of water present to the quantianode element except through d open lower pol-tionv which ty of ca bon p esent s arie T e r ul obtained conclucomprises diluting said used sulfuric acid with water so that sively demonstrate the relationship be ween Wat r n n the total water content of said diluted acid is in the range of and carbon contentofthe acid charged to the electrolytic cell. 3,0 t 54 ti b i h h amount of carbon to b In example A, 440 grams of used alkylation acid or 93.4 removed from said diluted used sulfuric acid, passing said percent acidity from a plant scale alkylation unit was indiluted used acid to said isolated cathode compartment of said troduced into the electrolytic cell. The original charge acid electrolysis cell and thence to said anode compartment of said contained 2.8 percent by weight of water and 2.2 percent by electrolysis cell while passing a undirectional electric current weight ofcarbon which was present as carbon in the hydrocarthrough said diluted used acid between said anode and said bon contaminants in the acid. The cathode was a inch cathode, withdrawing acid of reduced hydrocarbon content diameter carbon and graphite rod and the anode consisted of from said anode compartment. platinum foil. THe acid was subjected to an electrical current 2. Process of claim 1 wherein said electrolysis is carried out of 0.54 amperes at 5-7 volts and a temperature of 60 F. over with a current density within the range of 200-2500 amperes a period of 195 hours. At the end of this time the concentraper square foot. tion of the acid had increased to over 100 percent, or fum- 3. Process of claim 2 wherein said electrolysis is carried out ing, and the carbon content had been reduced to 0.7 percent with a current density of from about 200 to 600 amperes per by weight. square foot.

TABLE 1 Length of run Stoicltiometric Carbon in Weight percent Acid Volt- Water amount basis wt. percent 2804 used in Temp. Current age Ampere initial in Current, Ex grams in F. in amps. range hours Hours wt. percent Total C (3 removed Initial Final Initial Final ellicieney A. 440 60 0.65 5-7 127 195 2. 8 0. 4 0. 0 2. 2 0. 7 03. 4 (r) 57 B 1. 240 75 2.0 10-20 135 68 12.6 1.0 1.1 4.3 0. 5 s0 5 98.3 60 12s 60 0.39-1.13 7-0 72.1 140 17. 3 1.4 1. 7 4.3 1. 0 so 5 06.5 e D 12s 0.1-1.5 3-3 88.5 20.0 1.5 1.0 4. a 0.15 811.5 05.0 53

1 Fuminiz Examples B, C and D were conducted using a similar 4. Process of claim 1 wherein said anode is selected from procedure under the specific conditions shown in table 1. the group ofplatinum,iridium,lead and lead dioxide. Water was added to the initial acid to bring the water content 5. Process of claim 1 wherein said cathode is selected from up to 12.6 percent, 17.3 percent, and 20.0 percent in examples 55 the group consisting of graphite, carbon, lead and tantalum. B, C and D respectively. In example A in which no water was 6. Process of claim 1 wherein said electrolysis is carried out added to the original used acid, the total water content of the with mixing of the contents of said anode compartment. acid was only 2.8.This is only about 40 percent in the 7. Process of claim 1 wherein said electrolysis is carried out stoichiometric amount required to oxidize all of the carbon. 60 at a temperature within the range of about 30 to 175 F. Since there was a deficiency of water, free sulfur trioxide was 8. Process of claim 7 wherein the said electrolysis is carried obtained with only about 70 percent of the carbon being out at a temperature ofabout 130 F. removed. in examples B, C and D in which water was added to 9. Process of claim 1 wherein the said sulfuric acid is spent bring the water content up to 1.0, and 1.3 and 1.5 times that sulfuric acid alkylation catalyst containing approximately 3 to required respectively, the final acidities after subjecting to 5 percent by weight each ofcarbon and water. electrochemical action were 98.3 percent, 96.5 percent, and 10. Process of claim 9 wherein said spent sulfuric acid al- 95.0 percent respectively. In each case substantially all of the kylation catalyst is diluted with water in an amount such that acid charged to the cell was recovered, the only loss being the the total water content is approximately 3.0-5.4 times by hydrocarbon removed in the form of carbon dioxide. The weight ofthe carbon content. I results of examples B, C and D clearly show the value of ad- 11. Process of cla m 9 wh ere1n said electrolysis is conducted ding water to the i i order to b i a hi h i ld f hi h with a current density within the range of 200-2,500 amperes Concentration acid per square foot per hour for a sufficient length of time to Acids produced in examples B, C and D are satisfactory as remove Substantially all OfSaid fr f v charge acid to an alkylation system as catalyst for the alkyla- 12. Process of claim 1 wherein said electric current is about tion of isobutane with olefins. For such use it is preferred that 1 to 295 ampere hours per g t of a o e o dthe water content of the acid charged to the electrochemical process by adjusted such that the recovered acid contains not 13. Process of claim 12 wherein said recovered acid has a water content not in excess of about 2.0 percent by weight.

@1 3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, ,337 Dated October 26, 1971 Inventor) William B. Mather, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

651. 2, line 23, after "by" at the end of the line, insert --leads--.

Col. 2, line &3, change "1' to l l/3-.

Col. 3, line 1, change "2e' to --2e'-.

Col. 3, line 6, change "(CH )n" to --4(CH )n--. Col. 3, line 32, change the first "2H to "EH 0".

Col. 3, line 42, insert --2-- before "3 and change "280 to -4H SO Col. 3, line 17, after "VARIABLES" insert --IN THE PRocEss--.

Col. line 1 change "in" to --is--.

Col. L, line 69, change "preferably" to "preferable".

41 5 Table 1 5 line D, change Current efficiency from "53" to --52--.

Col, 5, line 76, change "by" to --be-.

Signed and sealed this 26th day of December 1972.

(SEAL) L Attest EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. Process of claim 1 wherein said electrolysis is carried out with a current density within the range of 200-2500 amperes per square foot.
 3. Process of claim 2 wherein said electrolysis is carried out with a current density of from about 200 to 600 amperes per square foot.
 4. Process of claim 1 wherein said anode is selected from the group of platinum, iridium, lead and lead dioxide.
 5. Process of claim 1 wherein said cathode is selected from the group consisting of graphite, carbon, lead and tantalum.
 6. Process of claim 1 wherein said electrolysis is carried out with mixing of the contents of said anode compartment.
 7. Process of claim 1 wherein said electrolysis is carried out at a temperature within the range of about 30* to 175* F.
 8. Process of claim 7 wherein the said electrolysis is carried out at a temperature of about 130* F.
 9. Process of claim 1 wherein the said sulfuric acid is spent sulfuric acid alkylation catalyst containing approximately 3 to 5 percent by weight each of carbon and water.
 10. Process of claim 9 wherein said spent sulfuric acid alkylation catalyst is diluted with water in an amount such that the total water content is approximately 3.0-5.4 times by weight of the carbon content.
 11. Process of claim 9 wherein said electrolysis is conducted with a current density within the range of 200-2,500 amperes per square foot per hour for a sufficient length of time to remove substantially all of said carbon.
 12. Process of claim 1 wherein said electric current is about 155 to 295 ampere hours per gram atom of carbon removed.
 13. Process of claim 12 wherein said recovered acid has a water content not in excess of about 2.0 percent by weight. 